This invention relates to alpha-olefin polymerization catalysts, and more particularly, modification of catalyst systems used in the preparation of normally solid, crystalline polyalpha-olefins with combinations of certain tertiary amines, organotin sulfides or hydrogen sulfide, and aromatic dicarbonyl compounds to improve the stereospecificity of the polymeric products without substantial decreases in catalyst activities.
It is well known that alpha-olefins can be polymerized using titanium trichloride-organoaluminum compound catalysts to form polymeric products which, while predominately crystalline, contain amorphous and low molecular weight components. In order to obtain a commercially useful polyalpha-olefin product, it is desirable to minimize levels of such amorphous and low molecular weight components in the product because the presence of these components not only leads to processing difficulties in the fabrication of products from the polymers, but also, the finished products are typically inferior in terms of physical properties. While it is well known that the alkane solubility of low molecular weight and amorphous materials produced during alpha-olefin polymerization affords a convenient method for removal thereof from the polymeric product, removal is disadvantageous for a number of reasons. For example, in slurry polymerization processes, removal not only entains extra processing steps, but the removed components are of little commercial value, and therefore, are typically discarded. Moreover, in bulk and gas phase polymerization processes there is typically no provision for removal of alkane soluble materials.
In view of these difficulties, it is desirable to modify titanium trichloride-organoaluminum compound catalysts such that levels of low molecular weight and amorphous components produced during polymerization are reduced. While a wide variey of catalyst modifiers are known to the art, care must be exercised in the selection and use of modifiers because reductions in levels of low molecular weight and amorphous components are typically accompanied by decreases in catalyst activity and polymerization rate. Accordingly, an effective catalyst modifier is one that minimizes the production of alkane soluble components without substantial decreases in catalyst activities.
Prior art proposals which may be considered relevant to the present invention include British Pat. No. 1,128,724, to Mitsubishi, which discloses alpha-olefin polymerization catalysts containing titanium trichloride, an alkylaluminum compound, and certain aromatic carbonyl compounds. It is disclosed that the use of the latter compounds as catalyst modifiers decreases the production of alkane soluble materials with only slight decreases, and in some cases, light increases in polymerization rate. Among the aromatic carbonyl compounds disclosed are dimethyl terephthalate and diethyl terephthalate. U.S. Pat. No. 3,367,923, to Tanaka et al., although not directed to reducing alkane solubles in alpha-olefin polymerization, discloses that polymers of controlled molecular weights and high impact strengths can be prepared by carrying out the polymerization in the presence of hydrogen and a catalyst comprising an organoaluminum compound, a Group IV to VI metal compound, at least one of such compounds being an alkoxy or aryloxy compound, and a third component which can be an aromatic carboxylic acid ester. Among the aromatic esters disclosed are methyl, ethyl, butyl, hexyl, octyl, decyl, and higher esters of dibasic aliphatic or aromatic acids including phthalic acid. The preferred esters are dibutyl, dioctyl, or higher diesters of phthalic acid. U.S. Pat. No. 3,950,268, to Karayannis et al. discloses polymerization catalysts containing a lower alkyl alkylaluminum or alkylaluminum halide component, a titanium trichloride, and an organotin sulfide, such as a lower alkyl bis-(trialkyltin)sulfide, or a mixture thereof with certain amine oxides. It is disclosed that such catalysts are effective in reducing levels of alkane solubles produced during alpha-olefin polymerization with only insubstantial effects on crystalline yields. U.S. Pat. No. 4,072,809, to Rogan discloses that reductions in alkane solubles can be achieved without seriously affecting yields through the use of catalyst compositions containing an alkylaluminum component, a titanium trichloride, and combinations of hydrogen sulfide with a sterically hindered, lower alkyl substituted, cyclic amine oxide or tertiary amine or with a trihydrocarbylphosphite.
Despite the above-identified prior art proposals, there exists a need to modify alpha-olefin polymerization catalysts to further reduce levels of alkane soluble materials produced during polymerization of alpha-olefins without substantial decreases in catalyst activities, particularly in view of the fact that in large scale polymerization operations even minor variations in solubles production and activities can have substantial economic effects. Accordingly, it is an object of this invention to provide such a method of modifying alpha-olefin polymerization catalysts. A further object of the invention is to provide an improved alpha-olefin polymerization catalyst. Another object is to provide a process for producing highly crystalline alpha-olefin polymers using such catalysts. Other objects of this invention will be apparent to persons of skill in the art from the following description and appended claims.
The objects of this invention can be achieved by modifying alpha-olefin polymerization catalysts with certain organotin sulfides or hydrogen sulfide, tertiary amines, and aromatic dicarbonyl components. The use of the modified catalyst compositions of this invention in the polymerization of alpha-olefins not only results in reduced levels of alkane solubles in the polymeric product as compared with polyalpha-olefins produced using unmodified catalyst compositions, but such reductions in solubles are achieved with only minor decreases, and in some cases, increases in catalyst activities. Additionally, it has unexpectedly been found that the use of catalyst compositions containing three-component modifiers according to this invention results in improvements in solubles reduction and catalyst activities which are superior to those achieved through the use of catalyst compositions containing the individual modifier components or two of such components in combination. Furthermore, the use of the invented catalysts is particularly advantageous in continuous slurry polymerization processes wherein polymeric product is separated from a polymerization medium which is subsequently purified, such as by distillation, and then recycled to a polymerization zone, because the low volatility of the aromatic dicarbonyl modifiers prevents the same from vaporizing during purification of the polymerization medium and re-entering the polymerization zone upon recycle of the medium.